With enhancement of performance of recent semiconductor devices having multilayer interconnection structures, there has been shown a tendency to use a low-permittivity insulating material in each of an interwiring insulating film and an intervia insulating film. In general, the low-permittivity insulating material has a high void ratio, and thus has a property of readily containing therein moisture. When a lot of moisture is contained in each of the interwiring insulating film and the intervia insulating film, a barrier metal covering wirings and vias may be oxidized by the moisture. When the oxidization of the barrier metal progresses, there is a possibility that even the barrier metal in a bottom surface of the via and the wiring itself covered with the barrier metal are oxidized to impede the electrical conduction, thereby causing deterioration of the electrical characteristics or the like. In particular, since a ratio of a surface area to a volume is larger in the via than in the wiring, a large influence of the oxidation of the barrier metal is exerted on the via.
A semiconductor device in which a plasma silicon oxide film is formed between an interwiring insulating film and an intervia insulating film is known as the related art in this field. This semiconductor device, for example, is disclosed in Japanese Patent KOKAI No. 2000-294634. According to this semiconductor device, it is possible to obtain a large etching selectivity between each of the interwiring insulating film and the intervia insulating film, and the plasma silicon oxide film. Therefore, the plasma silicon oxide film having a film thickness of, for example, several tens of nanometers can be used as an etching stopper.
However, according to the semiconductor device described in Japanese Patent KOKAI No. 2000-294634, since the plasma silicon oxide film has no function of blocking the moisture, there is a probability that the moisture in the interwiring insulating film moves to the intervia insulating film, so that not only the moisture in the intervia insulating film, but also the moisture in the interwiring insulating film promote the oxidation of the barrier metal covering the via. In addition, the plasma silicon oxide film has a large permittivity. Thus, in this case, it is feared to cause an increase in effective permittivity between the wirings.